He didn't. He got more than he paid for the Truck.
That's not a low offer
I have lived my 30 years of adult life, I am in my 50s, buying very old, and very cheap cars. $500-1500 dollars usually. I can say with some certainty that your son didn't do his homework and he doesn't know what to look for in a used vehicle.
I suspect he bought the Truck because of how it looked, not due to the mechanical nature.
Did he lose money? Yes. But that does not make the insurance offer low.